Only about three percent of all the water on the globe is fresh water. With expanding population there is increasing competition for fresh water and in many areas of the world there is a growing dependence on water purified by various means. Commercial methods used for water demineralization of brackish and sea water include reverse osmosis, electrodialysis and several different distillation methods. Plants processing up to 45 million gallons per day (MGD) are in use to produce drinking water from sea or brackish waters at a cost ranging from $1.31 to $1.75 per cubic meter for the largest plants.
In addition to being overused, existing fresh water resources are also subject to contamination by industry, mining and urban sewage. Billions of gallons of metal-bearing and cyanide-bearing wastes are generated annually by U.S. industry. Vast volumes of water contaminated with heavy metals have accumulated at such sites as abandoned mines. Typical metal contaminants include lead, zinc, iron, and copper at levels ranging from 1000 to 10000 parts per million (PPM) by weight. Other contaminants may include gold, silver, platinum, and molybdenum, with lesser quantities of manganese, nickel, arsenic, barium, cadmium and chromium. Inorganic contamination consists mainly of cyanide, sulfate, nitrate and fluoride. More than 3000 contaminated sites have been identified at U.S. Department of Energy (DOE) facilities located across the Nation Ref. 1!. At these sites the soil and groundwater are contaminated with hazardous and radioactive chemicals, which include numerous organic substances, heavy metals and radionuclides from the nuclear industry, including uranium, plutonium, cesium (.sup.137 Cs), strontium (.sup.90 Sr), technetium (.sup.99 Tc) and tritium (.sup.3 H).
Legislation has been enacted to clean up these sites and to prevent the generation of new wastes. Two basic waste management approaches have been taken; one is waste disposal and the other waste reduction and resource recovery by means of appropriate waste treatment processes. Because of the magnitude of the problem and its estimated cost of about $1 trillion by the year of 2000, numerous waste treatment technologies have evolved. In its effort to identify applicable processes the U.S. Department of Energy (DOE) Ref. 2! has recently evaluated 55 such technologies and recognized the following 24 general types: adsorption, bio-adsorption, chelation chromatography, coagulation, flocculation, distillation, evaporation, froth flotation, gas hydrate formation, neutralization, reverse osmosis, ultracentrifugation, anaerobic trickling filtration, biological reduction, chemical precipitation, sedimentation, dilution, electrochemical and electrokinetic methods, filtration, freeze crystallization, ion exchange, oxidation, solvent extraction, and ultrafiltration. From these technologies and those yet to be developed, a few may be selected as possibly cost-effective to deal with the cleanup problem. Description of numerous processes used for the recovery of heavy metals appears in the literature Ref. 2, 3, 4, 5, 6!.
The process of this invention falls into the electrochemical and electrokinetic category along with electrodialysis and the electrolytic recovery processes, which are briefly summarized next. Extensive evaluation of the electrolytic processes appears in three recent review articles Ref. 4, 7, 8!. One of these cited 136 publications for this type of separation process. Generally, in electrolytic recovery metal is plated onto the cathodes from which it is subsequently stripped off and recovered in a batch operation. The process is also amenable for isolation of components from complex mixtures by selective reduction of metal ions Ref. 9!. The metals which have been recovered by electrolytic methods include at least Ag, Au, Cd, Co, Cr, Cu, Ni, Pb, Pd, Pt, Sn and Zn. One of the most common applications is the recovery of copper from sulfuric acid solutions Ref. 10!.
Waste systems in which metal concentrations approach one weight percent can be handled with conventional commercial electrodeposition apparatus. Very low metal concentrations pose a problem with metal recovery because of the formation of a polarized layer around the cathode, which leads to a low rate of diffusion of ions into and across the layer, low deposition rate and limiting extraction of the metal from the water. This problem has been addressed effectively by the use of "high surface area" (HSA) electrodes made of carbon-fiber mats Ref. 11! or of porous metallic electrodes. The carbon-fiber electrode has a surface area which can be about 12,000 times greater than the apparent area, leading to an enhancement of the mass transfer by several orders of magnitude. A depletion of the metal from the solution to levels less than 1 mg/L has been achieved for silver, which satisfies the EPA requirements. A similar system has been used to recover up to 99.9% of cadmium from waste solutions Ref. 12!. The HSA system is most effective at concentrations of less than 10 mg/L Ref. 13, 14!. The HSA system is also used effectively to destroy cyanide by anode oxidation Ref. 12!.
The instant invention is more closely related to prior art which teaches a means of separation of ionizable constituents from liquid solutions by the action of electric current and the earth's gravity. One of the processes is known as electrodecantation, in which colloidal substances are separated from ionic solutions Ref. 15!. Electrodecantation was for a time a commercial process for separating rubber latex and certain biological components. A variation of this process, known as electrogravitation (EG), has been reported for the separation of ionic substances from aqueous solutions. Two types of apparatus have been described, one by Frilette Ref. 16! and the other by Murphy Ref. 17!. In the first type, described by Frilette, an electrodialysis cell was used, having vertically disposed end electrodes, with two or more, spaced-apart semi-permeable, parallel membranes in between. The separation of the solute occurs in two steps. The first step is the electrodialysis process achieved by electromigration, whereby enhancement and depletion of the original solution take place in-between alternate pairs of membranes. The second step, taking place under the action of gravitational force, is the separation of the concentrated solution from the depleted solution, which collect at the bottom and the top of the cell, respectively. Frilette Ref. 16! demonstrated desalination of 0.1N NaCl solution in a cell using synthetic ion exchange membranes. His results and additional data by Kollsman Ref. 18! indicate that a partial desalination can be achieved by this technique. Typical brackish waters of 2000-3000 PPM were demineralized to less than 500 PPM. A number of patents appeared on the subject Ref. 18, 19, 20!. Kollsman Ref. 18! has also shown that the symmetry of electrogravitational cell performance with membranes permits electrical reversal with no change in external hydraulics, which means that the low-density desalted product water will always rise to the top while the dense, concentrated solution will fall to the bottom of the cells.
In the second type of EG apparatus, described by Murphy Ref. 17! an electrolytic cell was used consisting of a cylindrical cell having vertically-disposed, concentric, cylindrical, reversible electrodes of Ag/AgCl, without membranes. He used long (minutes) dc current pulses of alternating polarity for the electrogravitation process. With the Ag/AgCl electrodes, the Cl.sup.- ion is released at the cathode, while the Na.sup.+ ion migrates towards it. In this manner a concentrated solution is formed near the cathode, which then sinks downward. At the anode the Cl.sup.- ions are depleted by the formation of AgCl, while the Na.sup.+ ions are depleted by the migration toward the cathode. In this manner a depleted solution is formed near the anode, which rises to the top. The combined opposite motion of the two liquids results in convection which is essential to the EG process. Thus, the EG process operates in conjunction with either electrolysis or electrodialysis. In the said prior art of the second kind which employs reversible Ag/AgCl electrodes, only those aqueous solutions containing the chloride ion as the anion have been successfully separated by the prior art. They included the salts KCl, NaCl and hydrochloric acid, HCl. This prior art does not teach the separation of other anionic species, nor does it teach the separation of cations. However, separation of the latter is of particular interest as in the case of hazardous wastes containing heavy metals and nuclear wastes. The same prior art Ref. 17! also fails to teach separation of the solute at concentrations of less than several hundred PPM, nor scaling to high capacity nor increasing the efficiency of the process.
Electrogravitation never became a commercial process, probably because it operated at a slow rate and because it was not effective at low concentrations of the solute of less than about 200 PPM Ref. 21!.
Separating constituents of a solution into fractions under the influence of an electric current and centrifugal force by means of a electrodialysis apparatus has been proposed by Kollsman Ref. 18! but never demonstrated. Kollsman apparently failed to foresee the potential of major improvements in the process that can be brought about by the use of the centrifugal force, nor did he teach the design of the required apparatus.
Other prior art includes application of centrifugal force to augment the separation of sewage sludge and simultaneous separation of platable metals by electrolysis as proposed in a German patent by H. Reimann Ref. 22!. In this proposed process a rotating device is used consisting of an external cylinder container which also acts as a cathode, and an internal concentric cylindrical post which is the anode. Sewage sludge is fed from the top end of the concentric device which has its axis positioned vertically. The sludge is collected on the cathode walls, while the metals are to be plated on the anode post, which is contrary to the electroplating practice and which appears to be incorrect. It also appears that in this equipment the centrifugal force would hinder rather than assist the removal of the heavy metals from the solution. Moreover, there is no provision for a continuous process; once a substantial layer of heavy metal would build up on the center post electrode, the process would have to be interrupted and the electrode would have to be replaced. Furthermore, this equipment is not suited for polarity reversal which is highly desirable for the continuous operation.
In a process named "centrifugal reverse osmosis desalination" Ref. 23! centrifuigal force is used to advantage in reducing the high pressure pump requirements. The reference stated that a 3000 gallon per day prototype of this process was undergoing tests on a Canadian Navy vessel.
There exists prior art that combines the action of electrical field and centrifugal forces in the separation process. U.S. Pat. No. 1,230,524 issued to Schwerin Ref. 24! describes a method of separating substances in suspension in a liquid, comprising adding a suitable electrolyte to the liquid and subjecting the liquid and the substance in sol-state (a colloidal suspension of a solid in a liquid) therein to the simultaneous action of centrifugal force and an electric current. The electrodes in this apparatus are disposed concentrically.
U.S. Pat. No. 1,558,382 issued to Marx Ref. 25! discloses a process for an electrocentrifugal separation of liquid or solid particles from liquids comprising simultaneous utilization of an electric field and centrifugal force in a centrifuge equipped with rotating concentric cylindrical electrodes. In the separation of colloidal suspension the particles having density greater than the fluid will impinge on the inner wall of the outer electrode and the less dense particles on the outer wall of the inner electrode. No provision is made for a continuous removal of the solids needed for a continuous operation.
U.S. Pat. No. 3,556,967 issued to Anderson Ref. 26! discloses a method and apparatus for separating macromolecular substances by electrophoresis in a liquid gradient while it is being stabilized in a centrifugal field. This apparatus or method are clearly not applicable for a continuous demineralization of aqueous solutions.
U.S. Pat. No. 4,008,135 issued to Gazda Ref. 27! discloses a method of deionizing a solution in an ultracentrifuge device also under the action of combined electric field and centrifugal force. Under the action of high centrifugal field, vaporization of the liquid takes place which facilitates the separation of the ions. In addition, electrolysis of the sodium chloride is said to take place with the formation of sodium hydroxide and chlorine. Reduction to practice in the form of desalination of a salt solution was described. However, the requirement of an ultracentrifuge precludes low-cost operation as well as purification of water on a large, commercial scale. Moreover, the vaporization of water and the electrolysis of salt are energy-intensive processes which would result in a high operational cost.
Prior art that is pertinent to the present invention is U.S. Pat. No. 3,196,095 issued to Wadsworth Ref. 28!. He discloses a method of purifying liquids by removal of solids in solution and particularly converting sea water to fresh water. This method comprises electrochemically concentrating solids in a portion of the liquid, leaving the rest of the liquid relatively free of the solids, and separating the concentrated portions from free portions by the action of centrifugal force. Wadsworth discusses removal of ions from fluid, giving as an example desalination of water, without providing any details or reduction to practice. Wadsworth also teaches continuous removal of ions that have been separated from the feed fluid.
While what he called an electrocentrifugal separation process is described in Wadsworth's patent, the design of the apparatus disclosed reveals serious flaws and deficiencies that would pre-empt any claimed effectiveness in the separation of dissolved solids from liquid solutions. A discussion of these deficiencies and their rectification by means of the instant invention is discussed later.
Other prior art, not involving either gravitational or centrifulgal forces is "electrochemical parametric pumping," described by Oren and Soffer Ref. 29a, 29b, 29c!. In this process ionic species, positive and negative are attracted to highly porous carbon electrodes via a double-layer capacitive process. By sequential charging, forward pumping, discharging, and reverse pumping, this process has been shown capable of high degree of separation of aqueous NaCl solutions at starting concentrations of 0.01 normal. The effluent in Oren and Soffer during charging is the diluent. During discharging the effluent is the concentrate.
Desalination by capacitive deionization has been reported involving the use of HSA aerogel electrodes Ref. 35, 36!. The apparatus and method is described in detail by Farmer in Ref. 36!. Neither involves the use of gravitational, centrifugal or Coriolis forces. Farmer's apparatus and method are similar to prior art described by Oren and Soffer in Ref. 29a, 29b, 29c!, referred to as `Electrochemical Parametric Pumping (EPP)`. The difference in Farmer's device is that he employs a a plurality of cells or stages, whereas Oren and Soffer used a single stage. However, Oren and Soffer expressly anticipated a multistage device for practical use.
The U.S. patent to Farmer, U.S. Pat. No. 5,425,858, discloses an apparatus that has cells connected electrically in parallel and intentionally directs fluid through the cells in a serpentine manner, which is a fluid flow in series between cells. The outflow for each stage is the feed for the subsequent stage. The concentrate is collected or rejected in each stage. Farmer also needs two systems in parallel to operate in any fashion of continuous operation. Farmer also uses constant voltage operation, and like Oren and Soffer mentioned above, the effluent during charging is the diluent, but during discharging is the concentrate. Farmer also employs low voltage at a high current.
Electromigration, which together with electrode processes is utilized in several of the deionization processes cited, as well as in the instant invention, is the transport of the dissolved ions through the liquid under the influence of applied electric fields, for the purpose of accumulating the ions at the electrodes of opposite polarity.
One means of accumulating them utilizes electrodialysis, described in Ref. 16 and 18. Another means is for the ions to undergo electrochemical changes such as oxidation or reduction, or simply, electrolysis. Still another way is to accumulate them by means of "double-layer" capacitive charging. The latter two means of accumulation are operative in the two different modes of deionization to be described further on, namely, the electrolytic mode and the electrostatic mode.
Gravitational and the centrifugal forces are utilized in several of the cited prior art as a means of separating the ionic species accumulated by electromigration from the depleted or purified liquid. In the instant invention centrifugal force along with Coriolis force are operative in the separation process. A brief discussion about these forces follows.
Gravitational force is referred to as the average force of earth's gravity on earth's surface. Gravity is commonly measured in terms of acceleration that the force imparts to an object on earth. The generally accepted average international value for acceleration due to gravity is 980.665 cm.multidot.s.sup.-2 (in English system, 32.17 ft.multidot.s.sup.-2). In the present description this value is also referred to as 1 G.
Centrifugal force is an apparent force associated with an object moving on a curved path, such as a ball on a string. The force that constrains the ball to move on the circular path is referred to as centripetal force, while the force that pulls on the string in an outward direction is centrifugal force. In this discussion centrifuigal force will also signify the force existing at any point at rest with the rotating system.
Coriolis force, also known as compound centrifugal force, and also as Coriolis acceleration, is additional force or acceleration acting on the motion of bodies in a rotating system. Thus, objects not at rest with the rotating system are subject to Coriolis force.
Wadsworth disclosed the use of electrodes preferably in the form of series of pairs of proximate conical sheet members or plates arranged co-axially. Under the combined action of electric field and centrifugal force, the concentrated, denser portion of the fluid is predicted by Wadsworth to flow along the radially inner wall of the conical electrode toward the periphery, while the depleted fluid is to flow on the radially outer wall of the opposite electrode toward the center. This situation may be possible in the case of an electrolytic mode and only at a constant d.c. polarity which Wadsworth teaches. Under periodically changing polarity, the concentrated, denser fluid would be formed also on the radially outer surface of the conical electrode. At this location the denser liquid would not flow along the electrode surface toward the periphery, but instead would traverse the space between the electrodes toward the opposite electrode, thereby becoming thoroughly mixed with the feed liquid and the depleted liquid. A similar situation would exist in the case of capacitive mode of operation where, as will be seen, dense liquids are formed on both electrode surfaces. The result of this shortcoming would be the absence of or very impaired desalination. It is to be noted that this problem would have surfaced had Wadsworth made an attempt to reduce his invention to practice. The present invention discloses an improved apparatus and method wherein such problems are circumvented as supported by specific examples.
The next deficiency in Wadsworth's patent is the crossover of the feed liquid and the concentrated liquid at the outer periphery of the conical electrodes, which causes further intermixing of concentrated and partially depleted fluids.
Another shortcoming of Wadsworth is the failure to constrain the fluids in between the electrodes from rotational motion. Such motion in fact takes place on account of the Coriolis force, which Wadsworth failed to take into consideration. Allowing the fluid to freely rotate also gives rise to further intermixing of concentrated, depleted and feed fluids. Wadsworth's patent also specifies that the feed liquid should flow from the bottom in between the electrode spacings weaving first toward the periphery, then toward the center and so on. In this manner, the Coriolis force would tend to accelerate the liquid in the forward direction when moving toward the center, then decelerate when moving toward the periphery, thereby causing still further mixing. The present invention not only constrains the fluids from free rotation but also takes advantage of the Coriolis force vector for optimum placement of the feed intake port, and the concentrate and the diluent and exhaust ports.
Still another weakness of the Wadsworth patent is that it specifies the use of continuous dc or pulsed dc current, apparently of the same polarity. This mode of operation would pre-empt the process from becoming continuous.
The present invention also specifically improves over prior art such as Oren and Soffer and the U.S. Farmer patent. Cells are connected in series but fluid flow is in parallel. Operation is continuous, without the need of a second system in parallel. Stages can be added radially or axially. Electrical connection between stages is preferably in parallel. Power input is high voltage and low current, which is more practical and cost effective.
Therefore, there is a real need in the art for improvements regarding separation of ionic substances from liquids. Waste-water treatment is a highly competitive field in which technological and cost advantages of a given process decide its success. The innovative features of this invention can be identified from the objects of this invention which include:
An improved apparatus and method for the separation of a variety of ionizable substances from liquid solutions, including but not limited to minerals, heavy metals, radioactive nuclides, and a variety of anions dissolved in water. PA1 An apparatus and method employing Coriolis force to maximize the rate of separation, the current and energy efficiency, and the level of purity of the treated liquid, such as water. PA1 An apparatus and method for separating components in a dynamic mode in which the process liquid is fed in and the depleted liquid and a concentrate flow out. This mode is suited for processing larger amounts of liquid, such as very large volumes needed for water desalination or the treatment of water wastes, such as surface mine water. Consequently, one of the objects of the invention for the dynamic mode is a capacity of water treatment of up to, but not limited to 1.5 million gallons per day (MGD) for a single mobile or transportable unit. PA1 A membraneless apparatus and process for the separation of a variety of ionizable substances, assisted by Coriolis force, to eliminate problems with fouling, downtime and additional expense. PA1 A continuous process, as opposed to a batch process to achieve cost-effective operation. PA1 Recovery of water of adequate purity for reuse, as for irrigation. PA1 Ability to concentrate the solute in a liquid form to facilitate recovery of valuable components or the disposal of hazardous substances. PA1 Process scalability for large-scale applications such as mine waste-water treatment. PA1 A system capable of being incorporated into fixed or mobile units for ex-situ surface waste-water treatment.
These and other objectives, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.